A fluid bed results when a gas is passed upward through a bed of small solid particles at a rate fast enough to lift these particles and to create turbulence. This motion of particles, which are usually made of aluminum oxide or silica oxide, is similar to that of a fluid, which can be seen schematically in the figure.
When gas is forced upward through small holes in a supporting plate, two forces meet to raise the particles: the buoyancy of the gas and the retarding force known as aerodynamic drag. Objects to be heat treated are immersed directly into the bed of particles.
For more information, click on the link below (subscription required). Then scroll to Figure 7.
R Schneider; R. Mesquita; W Schützenhöfer, Distortion in Tool Steels, ASM International, 2014, https://doi.org/10.31399/asm.hb.v04d.a0005958
